莫名其妙的悲伤莫名其妙的心痛
[mò míng qí miào de bēi shāng mò míng qí miào de xīn tòng]
The phrase means 'Inexplicable sadness, inexplicable heartache.' This user may be describing their emotions which often come out of nowhere or for reasons unknown, pointing toward feelings of depression or sorrow that lack concrete cause.